feat: DuckDB quantiles in-process (gcc CGO), not Ladybug. (#34)

OQ3: internal/duckstats + bin/qa/stats.go. Zig stays Ladybug-only.
mikefarah/yq for small structured slices. Gitea #30.

skills/duckdb/SKILL.md

@@ -0,0 +1,38 @@
+---
+name: duckdb
+description: >-
+  Use https://github.com/duckdb/duckdb-go in-process for columnar analytics
+  (quantiles, GROUP BY, JSON/CSV/Parquet/JSONL scans) when that is faster than
+  nested Go loops. Not Ladybug. Not the web-search sqlite cache. Use when
+  aggregating samples, counting JSONL, or SQL over tabular files.
+---
+
+# duckdb-go
+
+Use https://github.com/duckdb/duckdb-go where it makes sense to get better performance in code.
+
+In-process DuckDB (`internal/duckstats`, `database/sql` driver `duckdb`).
+Vectorized SQL over tables, JSONL, CSV, Parquet. CGO with bundled libs
+(linux/darwin amd64/arm64). Links with **gcc/g++** (libstdc++), not Zig.
+D21 Zig (`bin/cgo/zcc`) is Ladybug/tokenizers only. After
+`eval "$(bin/cgo/zig env)"`:
+
+```bash
+CC=gcc CXX=g++ CGO_CFLAGS= CGO_LDFLAGS= ./bin/qa/stats.go <<< '[1,2,3,4,5]'
+CC=gcc CXX=g++ CGO_CFLAGS= CGO_LDFLAGS= go test ./internal/duckstats
+```
+
+| Store | Job |
+|-------|-----|
+| Ladybug | graph + FTS + HNSW (facts/info) |
+| modernc sqlite | web-search KV cache + throttle |
+| duckdb-go | OLAP: quantiles, counts, scans of many rows/files |
+| mikefarah/yq | small YAML/JSON/XML/CSV/TOML/HCL slice, not bulk |
+
+```bash
+./bin/qa/stats.go <<< '[1,2,3,4,5]'
+./bin/qa/stats.go --jsonl path/to/rows.jsonl
+```
+
+Do not open Ladybug through DuckDB. Do not put secrets or client PII into
+DuckDB files under the repo.